Cheliceriformes

One of the major subdivisions of the arthropods, including the arachnids, horseshoe crabs, and related forms. In these the body is divided into an anterior prosoma (cephalothorax), composed of eight segments plus a presegmental acron that usually has eyes, and a posterior opisthoma (abdomen) composed of twelve segments plus a postsegmental telson. As in other arthropods the mouth lies between the second and third segments, but whereas in other groups there is usually a pair of antennae on the last preoral segment, here there are none. The appendages on the prosoma are as follows:

1. None
2. None
3. Chelicerae (pinchers)
4. Legs or pedipalps
5. Legs
6. Legs
7. Legs
8. Legs

The legs on the prosoma are either uniramous or have a very reduced gill branch, and are adapted for walking or swimming. The appendages on the opisthoma, in contrast, are either absent or are reduced to their gill branch.

The Cheliceriformes are divided into three classes:

• Merostomata? ([horseshoe crab]?s, sea scorps - extinct)
• Arachnida? (spiders, scorpions, etc)
• Pycnogonida? (sea spiders)

The last of these actually have very strong differences from the body plan described above, but are placed here because they have chelicerae and do not show any clear relationship to any other arthropods. The Merostomata are found in the fossil record from the Ordovician onwards, and when young show some resemblance to the Trilobita, suggesting a possible relationship between these and the Cheliceriformes.